This, of course, hints at one of the most major twists in the entire series: the reveal in the final installment that Snape has been on Harry's side the entire time because of his love for Harry's mother, Lily, who was killed by Lord Voldemort.
Despite his attraction to the Dark Arts and Voldemort's ideology of wizard supremacy, Snape's love for Muggle-born Lily Evans, Harry's mother, eventually compelled him to defect from the Death Eaters. He then became a double agent for Albus Dumbledore and the Order of the Phoenix.
Severus Snape was on his own side, really. That meant he allied with Dumbledore because he was planning to do what Snape deemed right and proper. It may look like Dumbledore is controlling Severus, but he was way too brilliant of a wizard to allow anyone to control him.
Knowing that Snape has been on the Dumbledore's side for all seven of the Harry Potter books reveals him as an extremely brave man and a truly skilled spy. Snape has had to face and deceive the terrifying Lord Voldemort constantly.
Protecting him against Quirrell
During the Quidditch match, Harry believes that Snape is trying to hex him, but in reality, Snape is trying to counteract the hex placed on Harry by Quirrell. It's clear that Snape has been secretly working behind the scenes to help Harry since the very beginning.
Now, as firmly established, Snape was not the greatest fan of Harry, but that didn't mean that he ever stopped loving Lily. Dumbledore was surprised that Snape seemed to care for the boy. With a swish of his wand, Snape conjured up a Patronus – Lily's Patronus, a doe. 'Always,' he said.
The acclaimed author explained that Harry paid tribute to Snape because of "forgiveness and gratitude." Though Snape bullied Harry, he also saved him.
“You disgust me,” said Dumbledore, and Harry had never heard so much contempt in his voice. Snape seemed to shrink a little, “You do not care, then, about the deaths of her husband and child? They can die, as long as you have what you want?” Snape said nothing, but merely looked up at Dumbledore.
He doesn't trust Snape as Dumbledore does, but he believes that he can detect any disloyalty. Moreover, Voldemort regards Snape as highly intelligent and therefore ' as a true Slytherin ' a person whose top priority is to take care of himself. To his twisted mind, selfless equals stupid.
Similarly to Harry, Snape's patronus represents a person. In the memories he gives Harry before his death, Snape shows Dumbledore his patronus, using the form to prove his love of Lily Potter, Harry's mother. This indicates that Lily also had a doe patronus.
So even if she isn't told explicitly, McGonagall is intelligent: she knows that Snape works for Dumbledore, and pretends to work for Voldemort, so he must be a double agent. It's very unlikely she knows why Snape defected, as Snape makes Dumbledore promise not to tell anyone the true reason.
After her death, Dumbledore convinced him that the only way he could remain loyal to her and her memory was to keep her son safe. And D convinced Snape that the only way he could do that was to become part of Dumbledore's plan. So yes, Snape was loyal to D because he had no other way to be true to himself and to Lily.
Although Snape may not have projected his memory of James onto Harry, he still would have been predisposed to dislike him by virtue of him being James Potter's son. Harry being in Slytherin would not have changed this fact.
Snape was rude to Harry because he was a bitter old man who held the sins of his childhood bully against his son. And Harry was a rulebreaker who got away with a lot of weird stuff which only fueled Snape's bitterness.
So, Voldemort had to kill Snape for the wand's allegiance to switch. Voldemort knew he couldn't cast Avada Kedavra with the Elder Wand to kill its master, so he ordered Nagini to be the one who killed Snape. In reality, the Elder Wand belonged to Draco, as he was the one to disarm Dumbledore in the Astronomy Tower.
In order to preserve his position as a trusted Death Eater, Severus Snape was forced to tell Voldemort the true date at which the Order planned to move Harry.
He did not even wish he had chosen differently — he believed he had made the only reasonable choice. He felt what he said, regret. Because he had to kill his most useful servant. His other Death Eaters either fled or were imprisoned when Voldemort was 'half-dead'.
He was on the point of collapse when it happened,” Rowling said. “Dumbledore didn't want to lose his wand at that point and Draco disarmed him. So that meant that the wand gave Draco its allegiance, even though Draco never knew it, even though Draco never touched it.
For all intents and purposes, this act only solidified Harry's suspicions that Snape was working against Dumbledore the entire time. But there were parts of the story Harry didn't know, including that Dumbledore was already dying and had asked Snape to kill him when the time was right.
the 13th and final instance. at least by my count of Severus Snape helping save Harry. is when he shares his memories with him before dying near the end of the Deathly Hallows. these memories not only absolve Snape of his guilt in the death of Dumbledore.
Who would have guessed that Snape could love anyone, let alone Harry Potter's mother. He didn't protect Harry because it was the right thing to do; he didn't risk his life by being a spy for Dumbledore because he enjoyed living on the edge; he couldn't even stand Harry, yet Snape died protecting Harry's life.
When Snape used his Patronus to lead Harry to the Sword of Gryffindor. But of course, it was not just Harry whose Patronus saved the day.